The conventional RCA cleaning s for semiconductor substrates consists of two steps involving different solutions: an alkaline solution, the so called SC1 solution and an acidic solution, SC2. The SC1 solution is composed of 1 part ammonia (NH4OH), 1 part hydrogen peroxide (H2O2) and 5 parts ultra pure water (H2O) and is often referred to as APM-cleaning (i.e. Ammonia Peroxide Mixture). Originally, it was used to remove organic residues by oxidation. Later it has been proven to be very efficient to remove particles.
A drawback of the SC1 solution is that metallic contamination such as Fe and Cu are found to catalyze the decomposition reaction of the peroxide (see e.g. Mertens et al., Proc. of the 5th Internat. Symp. on Cleaning Technology in Semiconductor Device Manufacturing PV97-35 (1997)) leading to a decrease in the bath lifetime.
Chemical solutions comprising an oxidizing compound have often problems related to the stability of the solution. In pure form, aqueous solutions are stable over extended periods of time. However, the presence of certain metal ions in the solution causes decomposition of the oxidizing compound. Consequently, stabilizers to prevent such decomposition are preferably added. Stabilizers can include, e.g., a complexing compound, such that the complexing compound will bind to the metal, and consequently the metal is not available for reaction with the oxidizing compound. Thus, the decomposition of the oxidizing compound is substantially inhibited and the lifetime of the solution is increased.
Very stringent specifications must be met by oxidizing solutions for specialized applications such as semiconductor applications or reagent chemicals.
An overview of stabilizing oxidizing compound, and more specifically hydrogen peroxide solutions, is given in Kirk-Othmer Encyclopedia of Chemical Technology (4th edition), vol. 13 pg 965.
Another problem associated with SC1 cleaning solutions is that metals precipitate on silicon surfaces. Aluminum, iron and zinc especially have been shown to adsorb strongly on the wafer surface (see e.g. Mertens et al., Proc. of the 8th Internat. Symp. On Silicon Materials Science and Technology PV98-1 (1998)). In order to remove the metallic surface contamination, the SC2 solution consisting of 1 part hydrochloric acid, 1 part hydrogen peroxide and 6 parts ultra-pure water is used. However, it is expensive to obtain hydrochloric acid of sufficient quality for the usage in SC2 solution. There is also a risk of re-contaminating the surface with particles. Problems also occur in spray tools due the corrosive behavior of hydrochloric acid.
With the progress in semiconductor manufacturing the requirements concerning particle and metal contamination as well as roughness of the silicon surfaces became more stringent. This led to a number of variations of the RCA clean.
The potential problems related to the SC2 and the consideration to reduce process time and equipment by leaving out this acidic step led to the development of single-stage cleaning procedures. This can be done by using chemicals with reduced amount of metallic impurities. For that purpose, advanced purification procedures are established for obtaining ultra-pure water, ammonia and hydrogen peroxide. However, these chemicals are very expensive and the purity is not always assured when they are used in a cleaning bath. Moreover, the cleaning solution is not very robust with respect to metal contamination from the semiconductor substrate and from the hardware.
Besides this, an extra step in the cleaning cycle to remove residual metallic contamination implies extra hardware, e.g., a SC2-tank and a rinse tank need to be used, and more chemicals. Leaving out this extra step results in a reduction of the hardware cost and a reduction of the amount of chemicals used in the cleaning cycle.
U.S. Pat. No. 5,466,389 describes cleaning solutions containing a complexing agent such as EDTA in combination with a nonionic surfactant. However, these cleaning solutions suffer from the drawback of weak stability of EDTA in peroxide containing cleaning solutions. In addition, in general, nonionic surfactants cannot be rinsed off easily from the wafer surface and traces of organic contamination are left on the wafer surface.
U.S. Pat. No. 5,885,362 describes a method for treating a surface of a substrate with a surface treatment composition. The surface treatment composition comprises a liquid medium containing a complexing agent as a metal deposition preventive. The surface treatment composition is improved by incorporating at least two complexing agents. A first complexing agent is preferably an aromatic hydrocarbon ring with at least an OH or O− group bonded to a carbon atom constituting the ring. A second complexing agent is compound having a donor atom, in the molecular structure.
U.S. Pat. Nos. 5,290,361 and 5,302,311 describe an aqueous hydrogen peroxide solution further comprising a complexing compound containing phosphonic acid groups and showing complexing ability. Cleaning solutions comprising phosphonic acid groups are not effective because enhanced deposition of Cu has been measured. In addition, there is always a risk of leaving P-contamination on the wafer surface which makes the cleaning solutions less suitable.
U.S. Pat. Nos. 5,280,746 and 5,840,127 describe the use complexing agents with hydroxamate functional groups. However, these complexing agents have limited stability in cleaning solutions containing peroxide.
U.S. Pat. No. 6,066,609 describes an aqueous cleaning solution comprising a base, hydrogen peroxide and a complexing agent being a crown ether with sidegroups able to complex metallic species. However the phosphonic acid side groups may also contribute to unwanted P contamination on the wafer surface. In addition, these complexing agents show a limited stability and a lower metal removal performance.